1. Field of the Invention
This invention concerns a process for exothermic heterogeneous synthesis in which the synthesis gas flows over a series of catalytic beds superimposed but separate one from the other contained within the same reaction space, all the reacted gas collects in the central zone of the last lower catalytic bed and from here it flows upwards to the top of the space inside which its heat is exchanged and steam is produced.
The invention also concerns reactors to put this process into effect, consisting of a pressure-resistant external shell, of baskets of catalytic beds all inside the same shell, of a cartridge and of a heat exchanger.
2. Description of the Related Art
In a recent patent application it was pointed out that in ammonia production a remarkable amount of heat is developed in the synthesis reaction N2+3H2, which is generally recovered for the final purpose of producing steam recycled to reduce energy consumption.
The most advanced technology tends towards maximum recovery of said synthesis heat at the highest possible thermic level; synthesis units and their main component, the reactor, are therefore designed to this end.
The reactors used in new plants have several catalytic beds with intermediate quenching of the gas by means of indirect exchange through heat exchangers; moreover part of the reaction heat is removed with an external cooling fluid such as for example water feeding a boiler or by means of generating steam before the last reaction stage, and this for the purpose of being able to operate at the highest possible temperature (heat recovery at maximum thermic level) without any limitations of the greatest possible efficiency obtainable.
Maximum temperature and maximum yield are in fact contrasting needs as amply shown by the relevant diagrams which indicate in abscissa the concentration of ammonia and in ordinate the temperature of the gas.
Major synthesis reactor designers in general have favored reactors with several catalytic beds in at least two distinct parts in series, in order to satisfy the above-mentioned need for the optimal exchange of reaction heat (at the highest thermic level) without limiting the maximum yield obtainable (Fertilizer Focus October 1987). Where two distinct parts of equipment are adopted, the first of the two reaction devices generally contains two catalytic beds with indirect intermediate quenching with an internal heat exchanger, while the second one generally contains a single catalytic bed.
Heat exchange between the two parts of the installation is carried out by introducing a boiler to produce steam. This is the case with the Topsoe Series 250 (Series 200 + Series 50) reactor and with the Uhde reactor, both of them with radial flow of the gas in the catalytic beds (Fertilizer Focus October 1987, pages 36 and 39).
There are also reactors in three distinct parts, each part containing a catalytic bed with axial gas flow as found in the C.F. Braun design (Nitrogen Conference, Amsterdam 1986). In this case a steam-producing boiler is inserted between the second and the third part of the installation (Nitrogen Conference, Amsterdam 1986, Mr. K. C. Wilson, Mr. B. J. Grotz and Mr. J. Richez of CdF Chimie).
According to a recent patent by C. F. Braun (UK Patent Application 2132501A), the gas/gas exchanger between catalytic beds, usually conveniently situated inside the reactors with at least two beds inside a single installation, is situated outside the reaction apparatus directly connected to the bottom of the shell containing a single catalytic bed. To minimize the problems of pipes at a high temperature, the tube connecting the above horizontal exchanger with the shell containing the catalytic bed is quenched with the fresh gas fed to the reactor.
After having pre-heated the fresh feed gas, the gas leaving the catalytic bed leaves from the exchanger and feeds the device containing the second catalytic bed (C. F. Braun reactor with several reaction devices as shown in FIG. 5 of the Wilson, Grotz, Richez report of the above-mentioned reference and at page 48 of Fertilizer Focus, October 1987).
The problem solved in the C. F. Braun patent mentioned above, i.e. avoiding contact between high temperature gas and the tubes connecting shell and exchanger, does not affect reactors with several catalytic beds within a single piece of apparatus since, as described above, the gas/gas exchanger is inserted directly inside the reactor itself.
Even according to C. F. Braun the problem of optimal heat exchange is solved in a complex way by introducing a boiler connected by means of complex piping to the reactor itself (see FIG. 5 of the C. F. Braun presentation, Nitrogen '86 and Fertilizer Focus October 1987, page 48).
All the above plans, although resolving the thermodynamic problem, are very complex, hence very expensive. Ammonia synthesis reactors operate in fact at high pressure, generally not below 80 bar, and more often between 130 and 250 bar, and at a high temperature (400.degree..div.500.degree. C.). The connecting tubes for the various pieces of equipment necessary according to the drawings described above (as shown schematically in the above-mentioned references), operate under critical conditions (high temperature of the gas between the various reaction beds) and must therefore be made of special material and with long runs to minimize the mechanical stress resulting from thermic dilation. The situation is particularly complex in reactors according to C. F. Braun, in spite of the measures taken according to the C. F. Braun patent application, UK No. 2132501A.
In the above-mentioned UK patent application the Applicants have suggested a process and a reactor with several catalytic beds which do not suffer from the drawbacks described above, can be produced in a single piece, and permit the easy removal of reaction heat between catalytic beds, and more particularly before the last catalytic bed, so as to achieve maximum recovery of reaction heat at the highest thermic level, such heat being exchanged, for example, to pre-heat boiler water or to produce steam directly.
The hot gas reacted in the last catalytic bed but one is transferred, through a duct generally situated along the axis of the vertical reactor, directly to the heat exchange system (pre-heater or boiler), returning then directly to the reactor through a duct, either internal or external to the above-mentioned transfer duct, creating an airspace for the gas to run through, returning to the reactor, said gas feeding then directly the last catalytic bed with an axial-radial or radial flow either centrifugal or centripetal. Said gas, after reacting in the last catalytic bed, is transferred once again to the central or external part of the reactor, and leaves then from the bottom of the reactor.
This system works very well with reactors with a cylindrical shell with a substantially constant diameter, but would meet some difficulties with reactors having a graduated diameter shell.